Method for generating hydrogen and oxygen by steam electrolysis

ABSTRACT

The present invention relates to a method for generating hydrogen and oxygen adsorbates by steam electrolysis at 200 to 800° C. using an electrolysis cell ( 30 ) comprising a solid electrolyte ( 31 ) which is made of a proton-conducting ceramic and which is arranged between an anode ( 32 ) and a cathode ( 33 ), each of which comprises a proton-conducting ceramic, and the ratio of the electroactive surface to the geometric surface of each of which is equal to at least 10, said method comprising the following steps: circulating a current between the anode ( 32 ) and the cathode ( 33 ), wherein the density of the current is no less than 500 mA/cm 2 ; inserting water in the form of steam, which is fed under pressure to the anode ( 32 ); oxidizing said water in the form of steam at the anode ( 32 ), and generating highly reactive oxygen at the anode ( 32 ) after said oxidation; generating protonated species in the electrolyte ( 31 ) after said oxidation and migrating said protonated species in the electrolyte ( 31 ); and reducing said protonated species at the surface of the cathode ( 33 ) in the form of reactive hydrogen atoms.

The present invention relates to a method for generating highly reactivehydrogen and oxygen by steam electrolysis by means of aproton-conducting membrane.

Conductive ceramic membranes are today the subject of widespreadresearch to increase their performances; in particular, said membranesfind particularly interesting applications in fields such as theelectrolysis of water at high temperature for the production of hydrogenor the treatment of carbon gases (CO₂, CO) by electrochemicalhydrogenation. Patent applications WO2008152317 and WO2009150352describe examples of such methods.

Hydrogen (H₂) appears today as a very interesting energy vector, whichis likely to take more and more importance for treating among otherspetroleum products, and which could, in the longer term, advantageouslyact as a substitute for oil and fossil fuels, the reserves of which aregoing to decrease considerably over coming decades. In this perspective,it is nevertheless necessary to develop efficient methods of producinghydrogen.

Numerous methods for producing hydrogen, from different sources, havecertainly been described but many of said methods prove to be unsuitablefor massive industrial production of hydrogen.

In this context, the synthesis of hydrogen from the steam reforming ofhydrocarbons may be cited for example. One of the major problems of thissynthesis route is that it generates, as sub-products, importantquantities of greenhouse gases of CO₂ type. In fact, 8 to 10 tonnes ofCO₂ are released to produce 1 tonne of hydrogen.

Two challenges thus arise for future years: searching for a novel energyvector that can be used without danger for our environment, such ashydrogen, and reducing the quantity of carbon dioxide.

Technical-economic estimations of industrial methods now take thislatter piece of information into account. However, it mainly involvessequestration, in particular underground sequestration inanfractuosities that do not necessarily correspond to former oilreservoirs, which in the long run may not be without danger.

A promising way forward for the industrial production of hydrogen is thetechnique known as steam electrolysis, for example at high temperature(HTE), at moderate temperature, typically above 200° C., or instead atintermediate temperature comprised between 200° C. and 1000° C.

At the present time, two steam electrolysis production methods areknown:

According to a first method illustrated in FIG. 1, an electrolyte isused that is capable of conducting O²⁻ ions and operating attemperatures generally comprised between 750° C. and 1000° C.

More specifically, FIG. 1 schematically represents an electrolyser 1comprising a ceramic membrane 2, conducting O²⁻ ions, assuring thefunction of electrolyte separating an anode 3 and a cathode 4.

The application of a potential difference between the anode 3 and thecathode 4 leads to a reduction of the steam H₂O on the side of thecathode 4. This reduction forms hydrogen H₂ and O²⁻ ions (O_(O) ^(X) inthe Kröger-Vink notation) at the surface of the cathode 4 according tothe reaction:

2e′+V_(Ö)+H₂O→O_(O) ^(X)+H₂

The O²⁻ ions, more specifically the oxygen vacancies (V_(Ö)), migratethrough the electrolyte 2 to form oxygen O₂ at the surface of the anode3, electrons e′ being released according to the oxidation reaction:

O_(O) ^(X)→½O₂+V_(Ö)+2e′

Thus, this first method makes it possible to generate at the outlet ofthe electrolyser 1 oxygen—anodic compartment—and hydrogen mixed withsteam—cathodic compartment.

According to a second method illustrated in FIG. 2, an electrolyte isused that is capable of conducting protons and operating at lowertemperatures than those required by the first method described above,generally comprised between 200° C. and 800° C.

More specifically, this FIG. 2 schematically represents an electrolyser10 comprising a proton-conducting ceramic membrane 11 assuring thefunction of electrolyte separating an anode 12 and a cathode 13.

The application of a potential difference between the anode 12 and thecathode 13 causes an oxidation of the steam H₂O on the side of the anode12. The steam fed into the anode 12 is thus oxidised to form oxygen O₂and H⁺ ions (or OH._(O) in the Kröger-Vink notation), this reactionreleasing electrons e⁻ according to the equation:

H₂O+2O_(O) ^(X)→2OH._(O)+½O₂+2e′

The H⁺ ions (or OH._(O) in the Kröger-Vink notation) migrate through theelectrolyte 11, to form hydrogen H₂ at the surface of the cathode 13according to the equation:

2e′+2OH._(O)→2O_(O) ^(X)+H₂

Thus, this method provides at the outlet of the electrolyser 10 purehydrogen—cathodic compartment—and oxygen mixed with steam—anodiccompartment.

More specifically, the formation of H₂ goes through the formation ofintermediate compounds, which are atoms of hydrogen adsorbed at thesurface of the cathode with variable energies and degrees of interactionand/or radical hydrogen atoms H. (or H_(Electrode) ^(X) the Kröger-Vinknotation). These species being highly reactive, they usually recombineto form hydrogen H₂ according to the equation:

2H_(Electrode) ^(X)→H₂

The patent application WO2008152317 has shown that the insertion ofsteam under pressure makes it possible to remain at moderate operatingtemperatures (of the order of 500 to 600° C.) while obtainingconductivity values assured by the movement of relatively high H⁺protons.

Nevertheless, this type of proton-conducting electrolysis is above allstudied and developed at the laboratory level with low current levels.Some fear, as in the case of O²⁻ conducting electrolysis, phenomena ofdelamination of the electrode, which can induce decohesion between saidelectrode and the electrolyte during use with higher current densities.

In fact, unlike the field of anionic-conducting electrolyses, the chargecarriers (protons) are not intrinsic to the structure of the membraneand are thus consequently more limited in the structure than the chargecarriers of an anionic conduction, which are formed by the vacancies ofthe structure.

Consequently, it is known to use in the field of anionic-conductingelectrolyses current densities at the terminals of the electrodesgreater than the current densities used in the field ofproton-conducting electrolyses.

Nevertheless, the application of such current densities at the terminalsof the electrodes of a proton-conducting electrolyser in electrolysersof the prior art would cause localised over-voltages which would lead tophenomena of delamination of the electrodes.

In this context, the present invention aims to propose a method forgenerating highly reactive hydrogen and oxygen adsorbates by steamelectrolysis using an electrolysis cell comprising a solidproton-conducting electrolyte, said method being able to beindustrialised while limiting the risks of delamination of theelectrodes.

To this end, the invention proposes a method for generating hydrogen andoxygen adsorbates by steam electrolysis at 200° C. to 800° C. using anelectrolysis cell comprising a solid electrolyte, which is made of aproton-conducting ceramic, said electrolyte being arranged between ananode and a cathode, said anode and cathode each comprising aproton-conducting ceramic and the ratio of the electroactive surface tothe geometric surface of each of which is equal to at least 10, saidmethod comprising the following steps:

-   -   circulating a current between the anode and the cathode, wherein        the density of the current is no less than 500 mA/cm²;    -   inserting water in the form of steam, which is fed under        pressure to the anode;    -   oxidizing said water in the form of steam at the anode;    -   generating highly reactive oxygen at the anode after said        oxidation;    -   generating protonated species in the electrolyte after said        oxidation;    -   migrating said protonated species in the electrolyte;    -   reducing said protonated species at the surface of the cathode        in the form of reactive hydrogen atoms.

It will be noted that the current may be continuous or pulsed; in thecase of a pulsed current, current density is taken to mean the currentdensity corresponding to the maximum value of the current intensityreached during the pulse.

The generation of the current may be obtained by different means:

-   -   a generator imposing a voltage at the terminals of the assembly        (i.e. a potential difference between the electrodes) may be        used;    -   a current source imposing a current between the electrodes may        be used;    -   operation in potentiostatic mode may also be used; in other        words, in addition to the two cathode and anode electrodes, at        least one third electrode known as reference is used. The        working electrode (preferentially the cathode) is then going to        be taken to a given potential with respect to the reference        electrode (in which one avoids passing too much current so as        not to modify its potential that serves as reference). The        generator making it possible to maintain automatically the        potential of the working electrode, even under current, is known        as a potentiostat.

As explained above, reactive hydrogen atoms are taken to mean hydrogenatoms adsorbed at the surface of the cathode and/or radical hydrogenatoms H. (or H_(Electrode) ^(X) in the Kröger-Vink notation).

Geometric surface of an electrode is taken to mean its flat externalsurface and electroactive surface is taken to mean the surfaceconstituted of the internal surface of the pores of the electrodewherein takes place the electrochemical reaction; in other words, it isthe internal surface on which the reaction takes place:2e′+2OH._(O)→2O_(O) ^(X)+H₂. The electrodes according to the inventionthus have a large number of triple points, namely points or contactsurfaces between an ionic conductor, an electron conductor and a gasphase.

The invention results from the finding made by the applicant that theuse of electrodes (cathode and anode) comprising a proton-conductingceramic (typically electrodes formed of a cermet including a mixture ofsaid ceramic of perovskite type and a metal alloy and/or a perovskitedoped with a lanthanide with one or more degrees of oxidation)surrounding a proton-conducting electrolyte and having a sufficientlyhigh electroactive surface/geometric surface ratio makes it possible towork at much higher current densities than those provided in the priorart without risk of delamination of said electrodes.

In fact, the consequent increase of the ratio between the electroactivesurface and the geometric surface of the electrodes compared to theratio of electrodes of the prior art makes it possible to reduce localover-voltages, which are responsible for phenomena of delamination ofthe electrodes.

The method according to the invention generates highly reactive hydrogenat the cathode of the electrolyser (particularly hydrogen atoms adsorbedon the surface of the electrode and/or radical hydrogen atoms).

These highly reactive hydrogen atoms H_(Electrode) ^(X) are formed atthe surface of the cathode according to the reaction:

e′+OH._(O)→O_(O) ^(X)+H_(Electrode) ^(X)

These highly reactive hydrogen atoms may be used as such for theproduction of hydrogen or for other applications that will be detailedhereafter.

The method according to the invention may also have one or more of thecharacteristics below, considered individually or according to anytechnically possible combinations thereof:

-   -   in a particularly advantageous manner, said ratio between the        electroactive surface and the geometric surface of said cathode        and anode is no less than 100; such a ratio makes it possible to        further improve the resistance of the electrodes at high current        densities without risk of delamination;    -   said density of the current is no less than 1 A/cm²;    -   the partial and relative steam pressure is advantageously no        less than 1 bar and preferentially no less than 10 bars;    -   the circulation of the current takes place between an anode and        a cathode, each made of a cermet constituted of a mixture of a        proton-conducting ceramic and a conducting material;    -   said conducting material is a passivable material with high        melting point being able to comprise at least 40% of chromium;    -   the circulation of the current takes place between an anode and        a cathode, each comprising a proton-conducting ceramic formed of        a perovskite doped with a lanthanide with one or more degrees of        oxidation, said ceramic being doped with a complementary doping        element taken from the following group: niobium, tantalum,        vanadium, phosphorous, arsenic, antimony, bismuth;    -   the method according to the invention comprises the following        steps:        -   introducing carbon dioxide CO₂ and/or carbon monoxide CO at            the cathode of the electrolysis cell;        -   reducing the CO₂ and/or CO introduced at the cathode from            said generated reactive hydrogen atoms;        -   forming C_(X)H_(y)O_(Z) type compounds, where x≧1,            0<y≦(2x+2) and 0≦z≦2x after the reduction of the CO₂ and/or            CO;    -   the method according to the invention comprises the following        steps:        -   introducing nitrogen containing compounds at the cathode of            the electrolysis cell;        -   reducing said nitrogen containing compounds introduced at            the cathode from said generated reactive hydrogen atoms;    -   said nitrogen containing compounds are NO_(x) type compounds        where x≧1, said method comprising a step of forming        N_(t)O_(y)H_(z) type compounds, where t is no less than 1, y no        less than 0 and z no less than zero, after the reduction of the        NO_(x);    -   said nitrogen containing compounds are N₂ compounds, said method        comprising a step of forming N_(x)H_(y) type compounds, where        x≧1 and y≧0, to result in the formation of NH₃ after the        reduction of the N₂;    -   said reactive hydrogen atoms are used to carry out a step of        hydrocracking at the cathode;    -   said reactive hydrogen atoms are used to convert aromatic        compounds at the cathode, for example into saturated alkanes        (paraffins) or into cycloalkanes (naphthenes);    -   the method according to the invention comprises a step        consisting in making said highly reactive oxygen react with a        compound introduced at the anode such that the latter undergoes        oxygenation.

The subject matter of the present invention is also an electrolysis cellfor the implementation of the method according to the inventioncomprising:

-   -   a solid electrolyte, which is made of a proton-conducting        ceramic;    -   an anode comprising a proton-conducting ceramic, said anode and        cathode each having a ratio between its electroactive surface        and its geometric surface equal to at least 10;    -   a cathode comprising a proton-conducting ceramic, said        electrolyte being arranged between said anode and said cathode;    -   means for inserting water in the form of steam, which is fed        under pressure to the anode;    -   means for inducing a current circulating between the anode and        the cathode, wherein the density of the current is no less than        500 mA/cm².

Said means for inducing a current circulating between the anode and thecathode may be a voltage, current generator or a potentiostat (in thiscase, the cell will also comprise at least one cathodic or anodicreference electrode).

Depending on the applications, the cell may also comprise means ofintroducing and evacuating pressurised gas in the cathodic compartmentand/or means of introducing and evacuating pressurised gas in the anodiccompartment.

Other characteristics and advantages of the invention will become clearfrom the description that is given thereof below, as an indication andin no way limiting, with reference to the appended figures, among which:

FIGS. 1 and 2, already described, are simplified schematicrepresentations of steam electrolysers,

FIG. 3 is a general simplified schematic representation of anelectrolysis cell for the implementation of the method according to theinvention;

FIGS. 4 to 6 are illustrations of applications using the cell of FIG. 3.

FIG. 3 represents in a general, schematic and simplified manner anelectrolysis cell 30, also known as elementary assembly, implementingthe electrolysis method according to the invention.

This electrolysis cell 30 has a structure similar to that of the device20 of FIG. 2. Thus, the cell 30 comprises:

-   -   an anode 32,    -   a cathode 33,    -   an electrolyte 31 formed of a proton-conducting electrolytic        membrane,    -   means 34 for inducing a current circulating between the anode 32        and the cathode 33, wherein the density of the current is no        less than 500 mA/cm²,    -   means 35 making it possible to insert under pressure steam pH₂O        into the membrane 31 via the anode 32 (the partial and relative        steam pressure of the current is no less than 1 bar and        preferentially no less than 10 bars).

It will be noted that the term partial and relative pressure heredesignates the insertion pressure compared to atmospheric pressure.

It will be noted that it is possible to use either a gaseous currentcontaining uniquely steam or a gaseous current containing partiallysteam. Thus, depending on the case, the term “partial pressure” willdesignate either the total pressure of the gaseous current in the casewhere the latter is uniquely constituted of steam or the partialpressure of steam in the case where the gaseous current comprises gasesother than steam.

According to a first embodiment, the anode 32 and the cathode 33 arepreferentially formed of a cermet constituted of a mixture of aproton-conducting ceramic and an electron-conducting passivable alloythat is able to form a passive protection layer so as to protect it inan oxidising environment (i.e. at the anode of an electrolyser). Thispassivable alloy is preferentially a metal alloy.

The passivable alloy comprises for example chromium (and preferentiallyat least 40% of chromium) so as to have a cermet having theparticularity of not oxidising at temperature. The chromium content ofthe alloy is determined so that the melting point of the alloy is abovethe sintering temperature of the ceramic. Sintering temperature is takento mean the sintering temperature required to sinter the electrolytemembrane so as to make it leak tight to gas.

The chromium alloy may also comprise a transition metal so as to retainan electron-conducting character of the passive layer. Thus, thechromium alloy is an alloy of chromium and one of the followingtransition metals: cobalt, nickel, iron, titanium, niobium, molybdenum,tantalum, tungsten, etc.

The ceramic of the anodic and cathodic electrodes 32 and 33 isadvantageously the same ceramic as that used by the formation of theelectrolytic membrane of the electrolyte 31.

According to an advantageous embodiment of the invention, theproton-conducting ceramic used by the formation of the cermet of theelectrodes 32 and 33 and of the electrolyte 31 is a perovskite ofzirconate type of generic formula AZrO₃ being able to be dopedadvantageously by an element A selected from lanthanides.

The use of this type of ceramic for the formation of the membrane thusrequires the use of a high sintering temperature in order to obtain asufficient densification to be leak tight to gas. The sinteringtemperature of the electrolyte 31 is more specifically defined as afunction of the nature of the ceramic but also as a function of thedesired porosity level. Conventionally, it is estimated that to be leaktight to gas, the electrolyte 31 must have a porosity level below 6% (ora density above 94%).

Advantageously, the sintering of the ceramic is carried out under areducing atmosphere so as to avoid oxidation of the metal at hightemperature, i.e. under atmosphere of hydrogen (H₂) and argon (Ar), oreven carbon monoxide (CO) if there is no risk of carburation.

The electrodes 32 and 33 of the cell 30 are also sintered at atemperature above 1500° C. (according to the example of sintering of azirconate type ceramic).

According to a second embodiment, the anode 32 and the cathode 33 mayalso be formed of a ceramic material that is a perovskite doped with alanthanide. The perovskite may be a zirconate of formula AZrO₃. Thezirconate is doped with a lanthanide that is for example erbium.Moreover, the perovskite doped with a lanthanide is doped with a dopingelement taken from the following group: niobium, tantalum, vanadium,phosphorous, arsenic, antimony, bismuth. These doping elements arechosen to dope the ceramic because they can pass from a degree ofoxidation equal to 5 to a degree of oxidation of 3, which makes itpossible to release oxygen during sintering. More specifically, thedoping element is preferably niobium or tantalum. Each electrode mayalso comprise a metal mixed with the ceramic so as to form a cermet. Theceramic comprises for example between 0.1% and 0.5% by weight ofniobium, between 4 and 4.5% by weight of erbium and the remainderzirconate. The fact of doping the ceramic with niobium, tantalum,vanadium, phosphorous, arsenic, antimony or bismuth makes it possible torender the ceramic electron-conducting. The ceramic is then a ceramicwith mixed conduction; in other words, it is conducting both toelectrons and to protons whereas in the absence of these dopingelements, perovskite doped with a lanthanide with a single degree ofoxidation is not conducting to electrons. Such a configuration makes itpossible to have electrodes made of a material of same nature as thesolid electrolyte, which has good conductivity both of protons andelectrons, and does so even when the ceramic is not mixed with a metal(as is the case of the first embodiment).

According to the invention, the electrodes 32 and 33 of the cell 30 aredesigned to have a ratio of their electroactive surface to theirgeometric surface that is equal to at least 10 and preferentially noless than 100.

Geometric surface is taken to mean the flat external surface of theelectrode, i.e. the surface receiving the flux of electrons.

Specific (or developed) surface is taken to mean the surface accessibleto a gas within the electrode: it is thus essentially constituted of theinternal surface of the pores.

Electroactive surface is taken to mean the part of the specific surfaceon which the electrochemical reaction takes place; in other words, it isthe internal surface on which the reaction takes place:

H₂O+2O_(O) ^(X)→2OH._(O)+½O₂+2e′

2e′+2OH._(O)→2O_(O) ^(X)+H₂.

According to the invention, the means 34 making it possible to inject acurrent circulating between the anode 32 and the cathode 33, wherein thedensity of the current is no less than 500 mA/cm² and preferentially noless than 2 A/cm² without risk of drop of current or delamination of theelectrodes.

The applicant has advantageously noted that the fact of using electrodesmade of a proton-conducting material and having a sufficientelectroactive surface (advantageously no less than 100) makes itpossible to increase notably the current density that can be usedwithout risk of delamination of the electrodes.

The determination of the ratio between the electroactive surface and thegeometric surface is carried out for example by means of a method ofcharacterising the porous surface of a cermet electrode detailed in thepublication “Characterization of porous texture of cermet electrode forsteam electrolysis at intermediate temperature”, C. Deslouis, M. Keddam,K. Rahmouni, H. Takenouti, F. Grasset, O. Lacroix, B. Sala,Electrochimica Acta 56 (2011) 7890-7898.

The general operation of the cell is described below.

The circulation of the current between the anode 32 and the cathode 33causes an oxidation of steam H₂O on the side of the anode 32. The steamfed under pressure into the anode 32 is thus oxidised to form oxygen O₂and H⁺ ions (or OH._(O) in the Kröger-Vink notation), this reactionreleasing electrons e⁻ according to the equation:

H₂O+2O_(O) ^(X)→2OH._(O)+½O₂+2e′

The H⁺ ions (or OH._(O) in the Kröger-Vink notation) migrate through theelectrolyte 31, to form hydrogen H₂ at the surface of the cathode 33according to the equation:

2e′+2OH._(O)→2O_(O) ^(X)+H₂

Thus, this method provides at the outlet of the cell 30 purehydrogen—cathodic compartment—and oxygen mixed with steam—anodiccompartment.

More specifically, the formation of H₂ goes through the formation ofintermediate compounds, which are hydrogen atoms adsorbed at the surfaceof the cathode 33 and/or radical hydrogen atoms H. (or H_(Electrode)^(X) in the Kröger-Vink notation). These species being highly reactive,

-   -   either they recombine to form hydrogen H₂ according to the        equation: 2H_(Electrode) ^(X)→H₂ (cf. FIG. 3);    -   or they react with other compounds injected on the cathodic side        33 (as will be seen with reference to FIG. 4 and following        figures).

It will be noted that the oxygen atoms adsorbed at the surface of theanode 32 may advantageously be used to carry out the production ofoxygen adsorbate O_(Electrode) ^(X) being able to be used in anoxygenation reaction at the anode, for example by injecting sulphurdioxide SO₂ or SO_(X) at the anode, which reacts with oxygen to formsulphuric acid H₂SO₄ or to form oxygen for the oxycombustion. One thushas for example the following equations:

H₂O+2O_(O) ^(X)→2OH._(O)+O_(Electrode) ^(X)+2e′

H₂O+SO_(X)+(3−x)O_(Electrode) ^(X)→H₂SO₄

O_(Electrode) ^(X)→½O₂

Concerning the operating temperature T1 of the device 30, the latterdepends on the type of material used for the membrane 31; whatever thecase, said temperature is above 200° C. and generally below 800° C., oreven below 600° C. Said operating temperature corresponds to aconduction assured by H⁺ protons.

FIG. 4 and following each illustrate a particular use of the cell 30 ofFIG. 3, in which highly reactive hydrogen is used to recombine withother compounds at the cathode 33.

FIG. 4 illustrates a first example in which the electrolysis cell 30 isused to form compounds of C_(X)H_(y)O_(Z) type, (where x≧1, 0<y≦(2x+2)and 0≦z≦2x) after the reduction of the CO₂ and/or CO.

The cell 30 of FIG. 3 further comprises means 36 making it possible toinsert under pressure gas (pCO₂ or/and CO) in the cathodic compartment33.

At the anode 32, the water is oxidised while releasing electrons whileH⁺ ions (in OH._(O) form) are generated.

These H⁺ ions migrate through the electrolyte 31 and are thus capable ofreacting with different compounds, which could be injected at thecathode 33, CO₂ and/or CO type carbon compounds reacting at the cathode33 with said H⁺ ions to form C_(x)H_(y)O_(z) type compounds (where x≧1,0<y≦(2x+2) and 0≦z≦2x) and water at the cathode.

The chemical equations of the different reactions may in particular bewritten:

(6n+2)H_(Electrode) ^(X) +nCO₂→C_(n)H_(2n+2)+2nH₂O

6nH_(Electrode) ^(X) +nCO₂→C_(n)H_(2n)+2nH₂O

6nH_(Electrode) ^(X) +nCO₂→C_(n)H_(2n+2)O+(2n−1)H₂O

(6n−2)H_(Electrode) ^(X) +nCO₂→C_(n)H_(2n)O+(2n−1)H₂O

The nature of the compound formed depending on the operating conditions,the overall reaction of formation of C_(x)H_(y)O_(z) may thus bewritten:

(4x−2z+y)H_(Electrode) ^(X) +xCO₂→C_(x)H_(y)O_(Z)+(2x−z)H₂O

The nature of the C_(X)H_(y)O_(z) compounds synthesised at the cathodedepends on numerous operating parameters such as, for example, thepressure of the cathodic compartment, the partial pressure of the gases,the operating temperature T1, the potential/current couple applied atthe cathode, the dwell time of the gas and the nature of the electrodes.

Concerning the pressure of the gases, the relative pressure of CO₂and/or CO of the current is no less than 1 bar and no more than therupture pressure of the assembly.

It should be pointed out that the total pressure imposed in acompartment—cathodic or anodic—may be compensated in the othercompartment so as to have a pressure difference between the twocompartments to avoid the rupture of the membrane assembly, electrodesupport if this has a too low rupture strength.

The operating temperature T1 of the device 30 also depends, in the rangebetween 200 and 800° C., on the nature of the C_(X)H_(y)O_(Z) carboncompounds that it is wished to generate.

FIG. 5 illustrates a second example in which the electrolysis cell 30 isused to reduce NO_(x) type compounds (x≦2) to form N_(t)O_(y)H_(Z) typecompounds (where t≧1, y≧0 and z≧0).

The cell 30 of FIG. 3 further comprises means 36 making it possible toinsert under pressure NO_(x) type compounds (x≦2) into the cathodiccompartment 33.

The problem consists in enabling the reduction by electro-catalytichydrogenation of the NO_(x) content of effluents produced for exampleduring the combustion of hydrocarbons or gas. The production of thesemolecules is 60% due to urban transport and 40% due to boilers andthermal power plants. These molecules easily penetrate the bronchiolesand affect respiration, causing hyper reactivity of the bronchial tubesin asthmatics, as well as increased vulnerability of the bronchial tubesto microbes, at least in children. Consequently, the regulations inforce require industries to limit their NO_(X) discharges.

It is known to those skilled in the art to reduce NO_(x) by two types ofmethod: selective non-catalytic reduction (SNCR) and selective catalyticreduction (SCR). Whatever the solution retained (i.e. with or withoutcatalyst), the latter is based on the use of ammonia to reduce theNO_(x) into N₂. These solutions have the drawback of using ammonia ashydrogen vector whereas it would be more interesting to directly treatthe NO_(x) with hydrogen. The production of ammonia assumes in factusing a method of steam reforming of hydrocarbons generating CO₂. Thismethod furthermore implies the use of a second reactor for theproduction of ammonia.

The method of using the cell 30 according to FIG. 5 is based on thefollowing principle: pressurised steam is introduced at the anodiccompartment 32 and the NO_(x) is fed under pressure at the cathodiccompartment 33. The incorporation under pressure of steam is going tolead to oxidation of this water in the form of steam at the surface ofthe anode so as to generate protonated species in the membrane which,after migration within the membrane, are reduced at the surface of thecathode into very reactive hydrogen capable of reducing by hydrogenationthe NO_(x) introduced into the cathodic compartment, such that theNO_(x) are reduced into less oxidised NO_(y) (where y≦x), then intonitrogen then into ammonia.

Thus, in the solution proposed, in a single reactor (i.e. the cell 30)are combined the generation of protons and the electro-catalytichydrogenation of NO_(x).

The monoatomic hydrogen adsorbates are formed at the surface of thecathode 33 according to the reaction: e′+OH._(O)→O_(O)^(X)+H_(Electrode) ^(X).

Consequently, in the presence of NO_(X) on the catholic side 33, thevery reactive adsorbates H_(Electrode) ^(X) react with the nitrogencontaining compounds at the cathode 33 to give reduced compounds ofnitrogen oxides of N_(t)O_(y)H_(z) type, where x≧1 and y≧0 and z≧0according to the reaction:

tNO_(x)+(2tx−2y+z)H_(Electrode) ^(X).→(tx−y)H₂O+N_(t)O_(y)H_(z).

As an example, these compounds are either NO_(y) less oxidised than theNO_(x) compounds fed in under pressure, nitrogen N₂, or ammonia NH₃.

The overall reactions at the electrodes are written:

H₂O+2O_(O) ^(X)→2OH._(O)+½O₂+2e′  (anode 32)

tNO_(x)+(2tx−2y+z)e′+(2tx−2y+z)OH._(O)→(2tx−2y+z)O_(O)^(X)+(tx−y)H₂O+N_(t)O_(y)H_(z)  (cathode 33).

The solution according to the invention makes it possible to reduce thenumber of reactors required for the reduction of NOx to a single andunique reactor accommodating the electro-hydrogenation.

FIG. 6 illustrates a third example in which the electrolysis cell 30 isused to produce ammonia by electro-catalytic hydrogenation of N₂. Itshould be noted that, according to this embodiment, it is also possibleto produce other N_(x)H_(y) type compounds where x≧1 and y≧0 beforeresulting in the formation of NH₃.

The cell 30 of FIG. 3 further comprises means 36 making it possible toinsert nitrogen N₂ under pressure into the cathodic compartment 33.

The problem is here to produce in massive quantity, at low cost andwithout emission of CO₂, ammonia, by electro-catalytic hydrogenation ofN₂.

At present, ammonia is produced by catalytic hydrogenation reaction ofN₂ during steam reforming of hydrocarbons. The synthesis of this productthus indirectly emits CO₂. In addition, the synthesis method induces avery great volatility of the production price of NH₃. In fact 80% of theprice of NH₃ is directly dependent on the price of the gas from which isproduced the hydrogen required for the synthesis. Thus, the volatilityof the price of ammonia is very high and dependent on that of the gas.

Moreover, according to known techniques, even when hydrogen is producedby electrolysis, it is necessary to use two reactors, one for theproduction of hydrogen and the other for the catalytic reaction.

The solution implemented in the cell 30 of FIG. 6 aims to produceammonia using a single reactor.

As previously, the hydrogenated monoatomic compounds are formed at thesurface of the cathode according to the reaction: e′+OH._(O)→O_(O)^(X)+H_(Electrode) ^(X).

Consequently, in the presence of N₂ on the cathodic side 33, veryreactive hydrogen H_(Electrode) ^(X) reacts with the hydrogenatedcompounds on the electrode 33 to give NH₃ according to the reaction:N₂+6H_(Electrode) ^(X).→2NH₃.

The overall reactions at the electrodes are written:

H₂O+2O_(O) ^(X)→2OH._(O)+½O₂+2e′

N₂+6e′+6OH._(O)→6O_(O) ^(X)+2NH₃

The solution of FIG. 6 makes it possible to reduce the number ofreactors required for the production of NH₃ (which serves as H₂ vector)to a single and unique reactor accommodating the electro-hydrogenation.

In the solution proposed, the hydrogen required for the reduction of thenitrogen is no longer produced from fossil energy; the method accordingto the invention is “cleaner” in so far as it does not generate CO₂.

Moreover, such a method makes it possible to do without the use ofcatalyst, which it is necessary to change and to recycle on account ofits deactivation by the water produced during the catalytic reductionreaction.

Finally, the solution proposed makes it possible to avoid a storage ofH₂ since the reactions of production of reactive hydrogen and reductionby hydrogenation take place in the same reactor.

As it has been possible to see with reference to FIGS. 4 to 6, thehighly reactive hydrogen produced by the cell 30 of FIG. 3 may be usedindustrially for very different applications. Obviously, the inventionis not limited to the embodiments that have been described. Thus, thehydrogenation by the highly reactive hydrogen atoms may also be used inthe petrochemical industry, for example to convert aromatic compoundsinto saturated alkanes (paraffins) and into cycloalkanes (naphthenes).The method according to the invention may also be used to carry outhydrocracking making it possible to convert, under hydrogen pressure andat sufficiently high temperature, heavy petroleum products into lightproducts: typically, hydrocracking makes it possible to obtain productssuch as diesel oil or kerosene from heavy residues.

1. Method for generating hydrogen and oxygen adsorbates by steamelectrolysis at 200° C. to 800° C. using an electrolysis cell comprisinga solid electrolyte which is made of a proton-conducting ceramic, saidelectrolyte being arranged between an anode and a cathode, said anodeand cathode each comprising a proton-conducting ceramic and the ratio ofthe electroactive surface to the geometric surface of each of which isequal to at least 10, said method comprising the following steps:circulating a current between the anode and the cathode, wherein thedensity of the current is no less than 500 mA/cm²; inserting water inthe form of steam, which is fed under pressure to the anode; oxidizingsaid water in the form of steam at the anode; generating highly reactiveoxygen at the anode after said oxidation; generating protonated speciesin the electrolyte after said oxidation; migrating said protonatedspecies in the electrolyte; reducing said protonated species at thesurface of the cathode in the form of reactive hydrogen atoms.
 2. Methodaccording to claim 1, wherein said ratio between the electroactivesurface and the geometric surface of said cathode and anode is no lessthan
 100. 3. Method according to claim 1, wherein said density of thecurrent is no less than 1 A/cm².
 4. Method according to claim 1, whereinthe partial and relative pressure of the steam is advantageously no lessthan 1 bar and preferentially no less than 10 bars.
 5. Method accordingto claim 1, wherein the circulation of the current takes place betweenan anode and a cathode each made of a cermet constituted of a mixture ofa proton-conducting ceramic and a conducting material.
 6. Methodaccording to claim 1, wherein said conducting material is a passivablematerial with high melting point being able to contain at least 40% ofchromium.
 7. Method according to claim 1, wherein the circulation of thecurrent takes place between an anode and a cathode each comprising aproton-conducting ceramic formed of a perovskite doped with a lanthanidewith one or more degrees of oxidation.
 8. Method according to claim 1,wherein it comprises the following steps: introducing carbon dioxide CO₂and/or carbon monoxide CO at the cathode of the electrolysis cell;reducing the CO₂ and/or CO introduced at the cathode from said generatedreactive hydrogen atoms; forming compounds of C_(X)H_(y)O_(Z) type,where x≧1, 0<y≦(2x+2) and 0≦z≦2x after the reduction of the CO₂ and/orCO.
 9. Method according to claim 1, wherein it comprises the followingsteps: introducing nitrogen containing compounds at the cathode of theelectrolysis cell; reducing said nitrogen containing compoundsintroduced at the cathode from said generated reactive hydrogen atoms.10. Method according to claim 1, wherein said nitrogen containingcompounds are compounds of the NO_(x) type where x≧1, said methodcomprising a step of forming compounds of N_(t)O_(y)H_(z) type, where tis no less than 1, y no less than 0 and z no less than zero, after thereduction of the NO_(x).
 11. Method according to claim 9 wherein saidnitrogen containing compounds are N₂ compounds, said method comprising astep of forming compounds of N_(x)H_(y) type where x≧1 and y≧0 to resultin the formation of NH₃ after the reduction of N₂.
 12. Method accordingto claim 1, wherein said reactive hydrogen atoms are used to carry out astep of hydrocracking at the cathode.
 13. Method according to claim 1,wherein said reactive hydrogen atoms are used to convert aromaticcompounds at the cathode.
 14. Method according to claim 1, furthercomprising a step consisting in making said highly reactive oxygen reactwith a compound introduced at the anode such that the latter undergoesoxygenation.
 15. Electrolysis cell for the implementation of the methodaccording to claim 1 comprising: a solid electrolyte, which is made of aproton-conducting ceramic; an anode comprising a proton-conductingceramic, said anode and cathode each having a ratio between theelectroactive surface and the geometric surface equal to at least 10; acathode comprising a proton-conducting ceramic, said electrolyte beingarranged between said anode and said cathode; means for inserting waterin the form of steam which is fed under pressure at the anode; means forinducing a current circulating between the anode and the cathode,wherein the density of the current is no less than 500 mA/cm².